I'd put her on a simple bit, like a snaffle (this might be do gentle). Sometimes, if horses don't respond to leg pressure, just carrying a crop (and not using it) can work quite well.
You say she's overweight? What is she eating? How often is she excersized?
If your worried she is going to flip in the arena, give her a good warm up, stay calm and relaxed, and try to get her as prepared as possible.
If you don't have time to excersize, or do not want to, how about lunging as a form of excersize?
Posted via Mobile Device